Page 1 of 9 

This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

Evaluation of the Potential for Internal 

Dose from Np-237 at the Rocky Flats 

Plant After 1983 

White Paper 

Rev. 0 

National Institute for Occupational 

Safety and Health 

December 30, 2014 

Dan Stempfley, Jim Bogard, and Chris Miles 

Oak Ridge Associated Universities Team

- 

Reviewed by LaVon Rutherford 

Division of Compensation Analysis and Support 
White Paper: Evaluation of the Potential for Internal Dose from Np-237 at the Rocky Flats Plant After 1983

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

INTRODUCTION

In the SEC Petition Evaluation Report for Petition SEC-00192 (SRDB 132777), NIOSH 

proposed the following class of workers to be added to the Special Exposure Cohort:

All employees of the Department of Energy, its predecessor agencies, and their contractors 

and subcontractors who worked at the Rocky Flats Plant in Golden, Colorado, from April 1, 

1952 through December 31, 1983, for a number of work days aggregating at least 250 work 

days, occurring either solely under this employment or in combination with work days within 

the parameters established for one or more other classes of employees included in the 

Special Exposure Cohort.

This recommendation derived, in part, from NIOSH’s conclusion that neptunium processing at 

the Rocky Flats Plant (RFP) took place at least until 1983, and that available monitoring data are 

insufficient for estimating potential internal exposures to neptunium (Np). Subsequent to that 

evaluation report, NIOSH has identified evidence that operations involving neptunium may have 

taken place after 1983, perhaps as late as 1988.

PURPOSE

This white paper summarizes NIOSH’s extensive research on neptunium-237 (Np-237)

processing at RFP after 1983 (the end of the current SEC period). It includes discussions of Np 

operations, inventories, available monitoring data, and an evaluation of the potential for internal 

exposure to Np-237 after 1983.

NATURE OF Np-237 OPERATIONS

A 1981 paper, Neptunium Processing at the Rocky Flats Plant (SRDB 24722, pdf p. 6), states 

that processing included preparation of pure neptunium oxide, metal, and metal alloys as well as

Np-237 recovery from a variety of residues. Processes employed included dissolution, anion 

exchange, precipitation, filtration, calcination, conversion to fluoride, and reduction to metal. 

Fabrication steps such as casting and rolling were also sometimes performed for the production 

of high-purity metal shapes and foils. Neptunium was recovered from residual materials 

including sand, slag, crucibles, casting skulls, and various alloys containing plutonium, tin, 

uranium, or zirconium. This description of RFP neptunium operations is echoed in a 1984

document, Actinide Processing at Rocky Flats (SRDB 24798, pdf p. 323).
White Paper: Evaluation of the Potential for Internal Dose from Np-237 at the Rocky Flats Plant After 1983

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with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

Np-237 OPERATIONS FROM 1962 - 1983

Neptunium was processed at Rocky Flats as early as 1962 (SRDB 24722, pdf p. 6). There is no 

evidence of continuous routine neptunium operations at Rocky Flats; rather, the evidence points 

to a series of discrete tasks performed from 1962 through 1983, involving a few grams to a few 

hundred grams, usually processed by request from other DOE laboratories.

The first special-order request for neptunium processing at RFP came from Lawrence Radiation 

Laboratory (now Lawrence Livermore National Laboratory [LLNL]), which required high-purity 

neptunium with gamma-emitting impurities removed. Techniques for purifying neptunium 

(including the removal of other actinides) were developed as a result of this project. The project 

also called for preparation of neptunium-plutonium (Pu) alloys, which were prepared by 

co-reducing Np and PuF4 mixtures with calcium metal; this effort led to a capability of 

producing pure Np metal that could be cast with Pu or uranium (U) to form alloys. The first 

Np-Pu alloy was produced in 1964, and several kilograms of Np metal were produced in 

subsequent years for preparation of U and Pu alloys. 

Other specific projects involving Np-237 processing included the preparation of high-purity 

neptunium oxide for the Oak Ridge National Laboratory (ORNL) isotope pool, neptunium metal 

foils for the Savannah River Plant, and neptunium metal disks for use in the liquid-metal, 

fast-breeder reactor, neutron dosimeter program (SRDB 24722, pdf pp. 6-7).

Based on reviews of numerous reports in the SRDB as well as reviews of classified documents, it 

is evident that only relatively small quantities of Np-237 were processed at RFP at any given 

time. The largest single operation appears to have been a special order in 1966 that involved just 

over 300 grams of neptunium (SRDB 126393). 

Np-237 OPERATIONS AFTER 1983

The current SEC period ends on December 31, 1983, as recommended by NIOSH in the 

SEC-00192 Petition Evaluation Report (SRDB 132777). Since that report was issued, NIOSH

has conducted an extensive search for evidence of Np processing after 1983. The result of this 

effort has been the identification of a single operation described in a 1987 document, 

Production-Scale Plutonium-Neptunium Separation and Residue Recovery at Rocky Flats Plant 

(SRDB 129512). This document does not make clear exactly when this operation took place, 

although it does indicate that the campaign lasted approximately one year. NIOSH interviewed 

one of the authors, who was also the Principal Engineer who designed the process and directed 
White Paper: Evaluation of the Potential for Internal Dose from Np-237 at the Rocky Flats Plant After 1983

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

the activities. He estimated that the operation began around January 1985 (SRDB 130877). This 

is the only post-1983 Np operation that NIOSH has been able to confirm. 

This 1985 operation involved the processing of plutonium scrap containing down to 0.5% (by 

weight) neptunium to separate and recover the two metals. The feed material for this process 

consisted of 63,728 grams of Pu containing 232 grams of Np. The separation process involved 

oxidizing the plutonium residue, passing Pu(III) through an anion exchange resin bed, and 

leaving Np(IV) behind for subsequent elution, evaporation, denitrification, and calcination to 

oxide (SRDB 129512, pdf p. 10). The authors reported completion of 24 separations over the 

course of a year using this process, resulting in the purification of 58,282 grams of plutonium by 

the removal of 222 grams of neptunium (~96% recovery). NIOSH re-interviewed the 

co-author/Principal Engineer who stated that project personnel consisted of five experimental 

operators who performed the glove box work and the Principal Engineer (SRDB: 2

nd interview 

transcript not yet ADC-cleared).

The final purified Pu that resulted from this operation contained only 0.0069% Np. The 

“neptunium product” consisted of 1429 grams of Pu and 222 grams of Np, a Pu:Np mass ratio of 

6.4 (SRDB 129512, pdf p. 11). This operation involved no purified neptunium; the dose from 

any internal exposure would have been dominated by the overwhelming amount of plutonium in 

the mixture, making neptunium bioassay unnecessary. Given the much greater specific activity 

of Pu-239, Pu bioassay would account for all organ doses, of which Pu would be the dominant 

component. An evaluation of the relative dose contribution from Pu and Np resulting from an 

inhalation intake of the Pu-Np mixture is presented later in the section titled Potential for Np-237 

Exposures After 1983.

According to the Principal Engineer and co-author of the 1987 report, separation of Pu and Np 

from alloys ended in 1987 (SRDB 130877, pdf p. 5). This statement is consistent with another 

reference that states that the neptunium program at RFP began in 1964 and was terminated in 

1988 (SRDB 33009, pdf p. 75).

Np-237 INVENTORY TOTALS AND MEASURED QUANTITIES

Documents captured by NIOSH and interviews conducted with former employees indicate that 

neptunium was present at RFP from 1962 to 2003. From 1963 through fiscal year 2003, 

neptunium quantities ranging between 29 grams and 1,318 grams are recorded in Material 

Balance Area accounts (SRDB 33009, pdf pp. 75-79; 126758; 131390). Neptunium-237 

inventory amounts at RFP from 1963-2003, obtained from the U.S. Department of Energy/U.S. 

Nuclear Regulatory Commission Nuclear Materials Management & Safeguards System 
White Paper: Evaluation of the Potential for Internal Dose from Np-237 at the Rocky Flats Plant After 1983

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

(NMMSS), are shown in Table 1. The neptunium program was reportedly terminated in 1988 

(SRDB 33009, pdf p. 75), although neptunium remained in the RFP nuclear material inventory 

until 2003. Nuclear material hold-up surveys were conducted sometime after 1990 in response 

to concerns by the Defense Nuclear Facilities Safety Board regarding nuclear MaterialUnaccounted-For at Rocky Flats. These surveys identified gram quantities of residual 

neptunium in previously-cleaned-out glove boxes located in Building 771, Rooms 153 and 182 

(SRDB 131225, pdf p. 6).

Table 1: RFP Np-237 Fiscal-Year-End Inventories

Fiscal

Year

Np-237

(g)

Fiscal 

Year

Np-237

(g)

Fiscal 

Year

Np-237 

(g)

1963 29 1976A* 468 1990 937

1964 601 1977 458 1991 941

1965 1,292 1978 567 1992 941

1966 740 1979 492 1993 941

1967 1,215 1980 744 1994 941

1968 972 1981 486 1995 941

1969 1,190 1982 699 1996 935

1970 1,105 1983 869 1997 926

1971 1,318 1984 1,040 1998 930

1972 788 1985 931 1999 880

1973 768 1986 985 2000 900

1974 470 1987 995 2001 886

1975 485 1988 970 2002 636

1976* 485 1989 935 2003** 501

Source: SRDB 33009, pdf p. 76; 126758; 131390

* Inventory values reported for FY-1976 and FY-1976A correspond with a change in the federal fiscal 

year to begin October 1. The U.S. budget year began July 1 prior to this time (SRDB 129510, pdf p. 2; 

129484, pdf p. 8). The values here agree with those reported elsewhere (SRDB 126758) for the June 30, 

1976 and September 30, 1976 quarterly inventory periods.

** Last reported inventory was on June 30, 2003.

It is difficult to draw hard conclusions about neptunium processing from the inventory table 

above. The data are presented primarily to show that the total Np inventory at RFP was always 

relatively low, typically less than a kilogram (compared to much greater inventories of plutonium 
White Paper: Evaluation of the Potential for Internal Dose from Np-237 at the Rocky Flats Plant After 1983

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with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

for the same time period). For example, the Rocky Flats TBD states that Building 771 recovery 

operations were processing 240 kg of Pu daily by 1965. This daily Pu throughput exceeds any 

single Np processing run by two to three orders of magnitude (SRDB 30013, pdf p. 15). A look 

at the year-to-year values reveals that inventory variability was higher in the earlier years and 

more stable from the mid-1980s onward.

INTERNAL DOSE MONITORING

Captured documents contain only two reported neptunium bioassays. A urinalysis result in July 

1966 was “Below Significant Level” (SRDB 24691, pdf p. 74; 26259, pdf p. 3). A urinalysis 

result in August 1966 reported 0.9 dpm/24h; a follow-up body count showed no detectable 

uptake (SRDB 24691, pdf p. 78; 26259, pdf p. 5).

WORKPLACE AIR MONITORING

NIOSH has found no neptunium-specific workplace air monitoring.

CONTAINMENT MEASURES EMPLOYED DURING Np-237 OPERATIONS

The neptunium processing descriptions described in the 1981 document, Neptunium Processing 

at the Rocky Flats Plant (SRDB 24722) supports NIOSH’s understanding that, by 1983, all Np 

operations were being performed in glove box facilities. For example, the document states:

A typical glove box for aqueous Np processing consisted of a “wet” section (for aqueous 

processes) and a “dry” section (for calcining precipitates and weighing powders) separated 

by an air lock. Each section had separate air inlet and exhaust filters. A door in the air lock 

was used to pass equipment and material between the wet and dry sections. A 1⁄8-inch-thick 

lead sheet was bonded to the stainless steel portion of the glove box and 1⁄4-inch leaded glass 

was placed over the glove box windows as a shield against gamma radiation from the 

neptunium and its impurities (SRDB 24722, pdf pp. 7-8). 

The single neptunium processing operation identified by NIOSH after 1983 involved the 

separation of neptunium from plutonium residues (see Np-237 Operations After 1983). 

According to the Principal Engineer who designed the process and directed activities, this 

operation was performed in glove boxes and tanks. The tanks contained feed material 

(plutonium and neptunium nitrate solution) that was piped directly into the glove boxes (SRDB: 

2

nd interview transcript not yet ADC-cleared).
White Paper: Evaluation of the Potential for Internal Dose from Np-237 at the Rocky Flats Plant After 1983

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

WORKPLACE INCIDENTS

No radiological incidents at RFP involving neptunium have been identified. 

Np-237 SHIPMENT AND RECEIPT DATA

NIOSH obtained and analyzed NMMSS records of neptunium shipments and receipts by Rocky 

Flats from 1983 until site closure in 2003 (SRDB 131384; 131387; 131388; 131389). Table 3 

shows the fiscal years during which grams of neptunium were received at RFP from other DOE 

sites (LANL, SRS, ORNL, and LLNL). No material was received after FY 1986. The 

three-letter designations in parentheses after the site acronyms are the DOE Reporting 

Identification Symbol (RIS) codes for the sites. Neptunium shipments from Rocky Flats to other 

DOE sites are shown in Table 4.

Table 3: Neptunium Receipts (g) by Rocky Flats

Fiscal Year

Originating Site

LANL 

(AUA)

SRS 

(DZA)

ORNL 

(FZC/FZG)

LLNL 

(LZB)

1983 --- 556 199 105

1984 --- --- 199 1

1985 99 --- --- 228

1986 1 113 --- ---

Table 4: Neptunium Shipments (g) from Rocky Flats

Fiscal 

Year

Receiving Site

LANL 

(AUA)

SRS 

(DZA)

ORNL 

(FZC/FZG)

Hanford 

(HRA)

LLNL 

(LZB)

Nevada 

Test Site 

(NAB)

WIPP 

(VPA)

Idaho 

Waste 

(VSB)

1983 --- --- 378 --- 220 40 --- ---

1984 --- --- --- --- 159 --- --- ---

1985 --- --- --- --- 146 26 --- 22

1986 73 --- --- 2 --- --- --- ---

1987 1 --- --- --- --- --- --- ---

1988 2 8 --- --- --- --- --- ---

1989 --- 36 --- --- --- --- --- ---

1990 --- --- --- --- --- --- --- ---

1991 --- --- --- --- --- --- --- ---

1992 --- --- --- --- --- --- --- ---

1993 --- --- --- --- --- --- --- ---
White Paper: Evaluation of the Potential for Internal Dose from Np-237 at the Rocky Flats Plant After 1983

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This working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

Table 4: Neptunium Shipments (g) from Rocky Flats

Fiscal 

Year

Receiving Site

LANL 

(AUA)

SRS 

(DZA)

ORNL 

(FZC/FZG)

Hanford 

(HRA)

LLNL 

(LZB)

Nevada 

Test Site 

(NAB)

WIPP 

(VPA)

Idaho 

Waste 

(VSB)

1994 --- --- --- --- --- --- --- ---

1995 --- --- --- --- --- --- --- ---

1996 --- --- --- --- --- --- --- ---

1997 5 --- --- --- 7 --- --- ---

1998 --- --- --- --- --- --- --- ---

1999 --- 46 --- --- --- --- 1 ---

2000 --- 18 --- --- --- --- --- ---

2001 --- --- --- --- --- --- 174 ---

2002 --- 297 --- --- --- --- 324 ---

2003 --- 394 --- --- --- --- 11 ---

POTENTIAL FOR Np-237 EXPOSURES AFTER 1983

In the SEC-00192 Petition Evaluation Report (SRDB 132777), NIOSH acknowledged that from 

1962 through 1983 there existed the potential for unmonitored worker exposures to Np-237. 

NIOSH further concluded that there was insufficient data available to estimate intakes of Np-237 

during that time period.

For the post-1983 period, NIOSH has identified only one operation involving neptunium (see 

Np-237 Operations After 1983). Workplace monitoring for this project included continuous air 

monitoring (CAM) and contamination surveys typical of the plutonium processing areas. 

Radiation Monitors (technicians) were available to provide radiological safety support. 

Personnel involved in the project were on bioassay programs typical of the plutonium processing 

environment (i.e., routine urinalyses and body counts) (SRDB: 2

nd interview transcript not yet 

ADC-cleared).

The previous discussion also noted that this single post-1983 operation did not involve purified 

Np, but rather Pu with Np as a contaminant. Even the most highly-concentrated Np product 

produced by this separation process was still mostly Pu, with a Pu:Np mass ratio of 6.4. Since 

the specific activity of Pu-239 is about 90 times greater than that of Np-237, the activity ratio of 

this Pu:Np mixture is greater than 500. As a result, all organ doses resulting from an intake from 

such a mixture would be dominated by the Pu component. The Np dose component would 
White Paper: Evaluation of the Potential for Internal Dose from Np-237 at the Rocky Flats Plant After 1983

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ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

comprise only about 0.1% for Type M Pu for most organs, and only about 1% for Type S Pu 

(SRDB 137075).

As mentioned previously, NIOSH conducted two interviews with the Principal Engineer/ 

co-author of the 1987 document, Production-Scale Plutonium-Neptunium Separation and 

Residue Recovery at Rocky Flats Plant (SRDB 129512). As Principal Engineer, he was closely 

involved with the 1985 Pu-Np separation operation under discussion. During those interviews, 

he stated that everyone who worked in a hot building had a urinalysis every few months as well 

as annual whole body counts (SRDB 130877; SRDB: 2

nd interview transcript not yet ADCcleared). If this individual or any of his co-workers had received Np intakes from this operation, 

the resulting organ doses would be adequately accounted for by the Pu bioassay program. Pu 

bioassay data are readily available for RFP claimants.

NIOSH has not identified any specific operations involving neptunium other than the 1985 

operation. If other operations of a similar nature (i.e., Np associated with Pu) did take place, Pu 

bioassay would have similarly accounted for any doses due to Np.

CONCLUSIONS

In conclusion, NIOSH finds no evidence that Np-237 intakes occurred at RFP after December 

31, 1983. If intakes had occurred during this period from the single identified Np operation, the 

resulting organ doses would be adequately accounted for by the available Pu bioassay data.

A recent interviewee provided additional names of individuals who may be knowledgeable about 

operations involving Np at RFP (SRDB: Interview transcript not yet ADC-cleared). NIOSH will 

follow-up on these leads and continue to evaluate any new information that becomes available 

regarding Np operations after 1983. If such information suggests that intakes of purified Np may 

have occurred during this time, NIOSH will reconsider its position, as appropriate.

For the 1962 through 1983 period, NIOSH maintains its existing conclusion, as stated in the

SEC-00192 Petition Evaluation Report (SRDB 132777), that unmonitored Np intakes may have 

occurred and insufficient data are available for estimating resulting doses.